As disclosed in U.S. Pat. No. 5,943,129, which is incorporated by reference herein, in fluorescent imaging, a sample is illuminated with excitation light of one wavelength while a resulting fluorescent emission at a second, typically longer wavelength is imaged. Because the fluorescent efficiency of many samples is low, i.e. typically 1 photon of fluorescent emission or less per 100 photons of excitation, the optical imaging system must efficiently collect the weak fluorescent emission without interference from the much stronger excitation signal. The optical system must provide an efficient optical path for delivering fluorescent emission light to the imager, with little or no such path for excitation light. Typically, spectral filters, such as colored-glass or interference filters, are used to provide at least some degree of the required wavelength selectivity. Disclosed herein is a low-profile sensor device that provides an efficient optical path for delivering emission light to the imager and a method for operating the device.